In the interdisciplinary field of tissue engineering, powerful new therapies are being developed to address structural and functional disorders of human health by utilizing living cells as engineering materials. In some areas of tissue engineering, researchers are creating two- and three-dimensional tissues and organs from combinations of cells in order to repair or replace diseased or damaged tissues.
Organ printing using inkjet printing is evolving to become more optimized by delivering multiple cell types and scaffolds to target specific regions. However, most current printing technologies are limited to hydrogel as the primary scaffold for tissue constructs. A major disadvantage of hydrogels is their low mechanical strength, which makes handling and in vivo application difficult, particularly for load-bearing implants. Alternative methods to create implants having enhanced mechanical properties are needed.
Inkjet printing technology is based on the rapid creation and release of liquid droplets, followed by their precise deposition on a substrate. Recently, this technology has generated increased interest in biomedical micro-fabrication, as it offers a practical and efficient method to dispense biological and/or material elements, including living cells (Boland et al., 2007, “Drop-on-demand printing of cells and materials for designer tissue constructs,” Materials Science & Engineering C-Biomimetic and Supramolecular Systems, 27(3), pp. 372-376; Xu et al., 2006, “Viability and electrophysiology of neural cell structures generated by the inkjet printing method,” Biomaterials, 27(19), pp. 3580-3588; Xu et al., 2005, “Inkjet printing of viable mammalian cells,” Biomaterials, 26(1), pp. 93-99; Xu et al., 2004, “Construction of high-density bacterial colony arrays and patterns by the ink-jet method,” Biotechnol Bioeng, 85(1), pp. 29-33).
The cell represents the basic unit of life and as such, it has become the focus of extensive research. Single-cell analysis is advantageous over conventional bulk cell methods as it allows complex and heterogeneous biological systems to be monitored at their most basic level (Shoemaker et al., 2007, “Multiple sampling in single-cell enzyme assays using capillary electrophoresis with laser-induced fluorescence detection,” Anal Bioanal Chem, 387(1), pp. 13-15). In recent years, single-cell based analytical devices have been increasingly applied in a wide range of biomedical applications, such as single-cell assays (Lu et al., 2004, “Recent developments in single-cell analysis,” Analytica Chimica Acta, 510(2), pp. 127-138), high throughput screening (Andersson et al., 2004, “Microtechnologies and nanotechnologies for single-cell analysis,” Curr Opin Biotechnol, 15(1), pp. 44-49; Brehm-Stecher et al., 2004, “Single-cell microbiology: tools, technologies, and applications,” Microbiol Mol Biol Rev, 68(3), pp. 538-559), single-cell protein libraries and gene expression (Fukuda et al., 2006, “Construction of a cultivation system of a yeast single cell in a cell chip microchamber,” Biotechnology Progress, 22(4), pp. 944-948; Janicki et al., 2004, “From silencing to gene expression: Real-time analysis in single cells,” Cell, 116(5), pp. 683-698), and miniature biosensors (Maruyama et al., 2005, “Immobilization of individual cells by local photo-polymerization on a chip,” Analyst, 130(3), pp. 304-310). These devices usually require the use of appropriate carriers to deliver and manipulate single cells.
Recently, microparticles that contain individual living cells have been applied in single-cell analytical systems as effective carriers (He et al., 2005, “Selective encapsulation of single cells and subcellular organelles into picoliter- and femtoliter volume droplets,” Anal Chem, 77(6), pp. 1539-1544). These particles provide easy handling of single cells and enhance detection efficiency (Huebner et al., 2007, “Quantitative detection of protein expression in single cells using droplet microfluidics,” Chemical Communications (12), pp. 1218-1220). Moreover, these particles have been used as micro-reactors to enhance and accelerate chemical and biochemical screening (Song et al., 2006, “Reactions in droplets in microfluidic channels,” Angewandte Chemie-International Edition, 45(44), pp. 7336-7356). This provides single cell analytical devices with new capabilities and improved detection efficiency (Taly et al., 2007, “Droplets as Microreactors for High-Throughput Biology,” Chembiochem, 8(3), pp. 263-272).
Currently, single-cell microparticles are mainly fabricated using microfluidic based methods (He et al., 2005, “Selective encapsulation of single cells and subcellular organelles into picoliter- and femtoliter volume droplets,” Anal Chem, 77(6), pp. 1539-1544; Huebner et al., 2007, “Quantitative detection of protein expression in single cells using droplet microfluidics,” Chemical Communications (12), pp. 1218-1220; Song et al., 2006, “Reactions in droplets in microfluidic channels,” Angewandte Chemie-International Edition, 45(44), pp. 7336-7356). However, these methods have some limitations. For example, micro-fluidic approaches are usually limited to specific geometries because they require laminar fluid flow to produce microparticles (Khademhosseini et al., 2004, “Layer-by-layer deposition of hyaluronic acid and poly-L-lysine for patterned cell cocultures,” Biomaterials, 25(17), pp. 3583-3592). These methods can only create small quantities of particles, since there are a limited number of micro-channels within these devices. Furthermore, the costly equipment, specialized material, and extensive expertise required for operation of these devices may further limit the use of these methods in single cell particle fabrication.
Thus, there is an acute need for more efficient approaches that can rapidly generate single cell microparticles with ease.